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6.4 - Energy
Work
Work – The process of moving an object by applying a force. Work = Force x displacement. Work is
measured in Joules (J) or Newton-meters (Nm).
Example: To prove his strength, a weightlifter pushes a refrigerator 2 meters across the floor by applying a
force of 200 N. How much work was done by the weightlifter? Answer: W = Fd = (200 N) (2 m) = 400 J
Note: To do work on an object, the object must move some distance as a result of the force. The object also
must move in the same direction as the object’s force. If the object does not move a distance or if that distance
is in a different direction than the direction of the force, then 0 J of work is done.
Energy – The property of an object or system that enables it to do work. Energy is measured in Joules (J).
Forms of Mechanical Energy
Mechanical Energy – The energy due to the positon or movement of an object. The two forms of mechanical
energy are potential energy (PE) and kinetic energy (KE).
Potential Energy – Stored energy by virtue of its position.
Gravitational Potential Energy – The stored energy in an object that is at some height above the ground.
PE = mass x gravity x height
Example: What is the gravitational potential energy of a 10 kg object 4 meters above the ground?
Answer: PE = mgh = (10 kg) (10 m/s2) (4 m) = 400 J
Kinetic Energy – The energy of an object in motion. KE = ½ mass x velocity squared
Example: What is the kinetic energy of a 4 kg book tossed across the room at 2 m/s?
Answer: KE = ½ mv2 = ½ (4 kg) (2 m/s)2 = 8 J
Example: What kind of energy does a box that is on a shelf of 2 meters above the floor have? Answer:
Gravitational potential energy
Kinetic Energy – The energy of an object in motion.
Example: When the box falls to the ground, what form of energy does it have? Answer: Kinetic energy
6.4 - Energy
Law of Conservation of Energy
Law of Conservation of Energy – Energy cannot be created or destroyed. It may reside in different forms, but
the total amount of energy never changes.
Example: The diagram above shows a roller coaster cart with 5000 J of potential energy at Point A.
(A) How much kinetic energy does the cart have at Point A? 0 J of Kinetic Energy (it is at rest)
(B) What is the total energy at Point B? 5000 J (Energy is conserved)
(C) How much potential energy does the cart have at Point B? 0 J of Potential Energy (no height)
(D) How much kinetic energy does the cart have at Point B? 5000 J (Energy is conserved)
(E) Where is the cart going its maximum speed? Point B (Most Kinetic Energy and least amount of
Potential Energy)
(F) What is the total energy at Point C? 5000 J (Energy is conserved)
(G) If the cart has 2000 J of potential energy at Point C, how much kinetic energy does it have?
PE = 2000 J. ME = PE + KE = 5000 J = 2000 J + KE. KE =3000J
Thermal Energy
Temperature – The measure of the average kinetic energy of the individual particles in matter. Objects that
have a high temperature have a high average kinetic energy. Temperature is measured in degrees Celsius (˚C)
or on an absolute scale of Kelvin (K).
Thermal Energy or Internal Energy – The total energy of all of the particles in an object. This depends on
the number of particles, the temperature of the object, and the arrangement of the particles. An object can have
the same temperature, but different energies. An example would be liquid water at 100˚C and water vapor at
100˚C. The water vapor has more internal energy than the liquid water.
Heat - The transfer of thermal energy from matter at a high temperature to matter at a lower temperature.
Note: Heat always transfers from the warmer object to the cooler object or from the high temperature matter to
the low temperature matter.
6.4 - Energy
Transfer of Heat
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Exothermic – Releasing Heat (-)
Endothermic – Absorbing Heat (+)
Heat is always transferred from the warmer object to the cooler object.
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Conductor – Transfers thermal energy well. An example would be metals.
Insulators – Does not transfer thermal energy well. An example would be rubber, wool, and wood.
States of Matter
Physical Change - Matter can change from one state to another when thermal energy is absorbed or released.
The attractions between molecules are changing but there are no bonds being broken. This is called a physical
change.
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Solids – Definite shape and volume. Particles have high levels of attraction, often in an orderly
arrangement. Not easily compressed.
Liquids – Arrangement of particles is not rigid or orderly. Definite volume but not shape. Not easily
compressed.
Gases – No definite shape or volume. Expands to fill the container. Low level of attraction, but high
amount of kinetic energy. Easily compressed.
Changes of States of Matter
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Melting – Solid to liquid (absorbing heat)
Freezing – Liquid to Solid (releasing heat)
Vaporization – Liquid to Gas (absorbing heat)
Condensation – Gas to Liquid (releasing heat)
6.4 - Energy
Phase Changes of Water – Note that when the phase is changing, the temperature is constant but there is still a
transfer of energy. The energy is going in breaking or forming attractions between molecules.